Available from the author: glass.ian at gmail.com or (for visitors to
SAAO) from the SAAO shops in Cape Town and Sutherland. Also from Ms Thembela
Matungwa tm@saao.ac.za

Price: R80 + postage

Mons Mensa Publishing, Cape Town (2008)

***100th Anniversary (2015)***

South Africa can claim a unique astronomical distinction: the first ever
measurement of the distance to a star. In 1833 Alpha Centauri was the first
star to have had its distance measured and it stayed the nearest star known
until 82 years later when a closer one was found. The new champion, another
South African discovery, was Proxima Centauri. To this day it remains the
record holder and Alpha the runner-up.

This book tells the story of the discoveries and the rather curious
characters involved in making them, starting with the first suggestion that
the distance to a star might be measurable and continuing right up to the
present-day investigations of Alpha and Proxima. The treatment is mainly
historical, with little that is mathematically challenging! Proxima
will be enjoyed by astronomers and non-astronomers alike.

Chapter 1: The distance of alpha Centauri

In 1820 the Royal Observatory
near Cape Town was founded to improve navigation by getting better positions
for the southern stars. By 1828 its instruments, among the most precise
constructed up to that time, were ready for action. In 1832, the bright
double star Alpha Centauri already formed part of the observing programme,
but acting on a tip from a soldier-turned astronomer, Manuel Johnson who
worked on the island of St Helena, Thomas Henderson and his assistant
William Meadows started observing it more intensively and soon had enough
readings to get its distance. Their work was afterwards refined by Thomas
Maclear.

Alpha Centauri has been known to be a double since 1689. William Herschel
showed that many doubles are in fact in orbit around one another and in the
19th century it became possible to measure the masses of its two components
using this fact. Over 400 observations of their relative positions have been
published and it now is one of the best-understood binaries. Late in the
19th century its distance was established at essentially it modern value by
Gill and Elkin at the Royal Observatory.

T. Maclear (Courtesy Prof B. Warner)

Chapter 3: Proxima

Eighty-two years after the Cape discovery, Robert Innes at the Union
Observatory in Johannesburg found a fainter star near alpha Cen that he
suspected was a third but somewhat separated member of the same system. Its
distance was measured over the following two years by Joan Voûte at the
Royal Observatory and by Innes himself, unknown to each other. Innes
prematurely declared it to be closer than alpha and named it 'Proxima
Centauri', or 'Proxima' for short. Voûte's better-quality observations
suggested it was no closer then alpha but, in fact, he was wrong and Innes
was right, largely by chance!

The certainty that it truly was the closest star had to wait until 1928
when Harold Alden, based at the Yale Southern Observatory in Johannesburg
with a superior telescope, made further measurements, though his results
were not accepted without controversy. It took many more observations and
finally the results from the Hipparcos satellite before the astronomical
world was absolutely convinced that Proxima really was the nearest star.

Proxima Cen colour composite

Proxima Centauri, the nearest star. Proxima is
stretched out and multi-coloured in this picture because it moved during the
long times between the three exposures that were required. Here, infrared
light is mapped as red, red light as green and blue light as blue. Besides
being the nearest star, Proxima is the 13th fastest moving one. It moves by
the diameter of the full Moon in about 500 years. By 2000, about 15 years
after these images were taken, it was at the position shown by the faint
cross.

Chapter 5: Modern studies of Alpha and Proxima

Because Alpha
and Proxima are the nearest stars of their types to us, they are still
observed with great interest. Their light can be analysed to find their
chemical compositions. The components of Alpha resemble our own Sun. All
three stars have been investigated for the presence of planets with
ever-increasing sensitivity. The methods used for making planetary searches
are described.

No certain planets have been found around either component of Alpha Cen
or Proxima as of 2015 although Dumusque et al (Nature Online, 17 October
2012) claimed the discovery of an approximately solar-mass planet in
orbit around Alpha 2 Cen with a period of 3.236 days. The planet-star
distance is only about 4% of the earth-Sun distance and implies that it will
be far too hot to support life. Their finding has, however, been received
with scepticism,

The 80-year orbit of the binary star Alpha Cen.

The periastron is the
closest they are apart and the apastron is the furthest.

Chapter 6: Bibliography

Suggestions for further reading and references.

Index

Readers' comments:

"Ian Glass's book on Proxima Cen is eminently readable, giving full
scope to the human foibles of the scientists who have worked on this
nearest of known stars, and showing how the process of science can
sharpen our understanding despite individual obstinacy, prejudice or
error. It follows efforts to answer seemingly simple questions about
Proxima and Alpha Centauri, explaining in clear, plain language how
the necessary scientific concepts work, and shows how and why we
still cannot say with certainty what the answers to some of these
questions are." - C.D. Laney

"Certainly the most complete account of man's epic quest for finding
the closest star that started in the 1830s, continuing all the way into the
space age. Intriguing is the fierce competition that existed between
astronomers to be the first to claim this achievement ..." - W.P. Koorts

"The distance to alpha Centauri was first measured by Henderson, an
astronomer at the then Royal Observatory (now SA Astronomical Observatory)
here in Cape Town in 1833 ... Ian Glass has written a short book on this and
the discovery of the nearest star to our Sun, Proxima Centauri. Simply
called Proxima, it is a wonderful story ..." - Case Rijsdijk,
Weekend Argus.

"This clear, interesting, and accessibly-written book explains how the
orbit of the earth in Copernicus's Sun-centred Solar System provides a
natural baseline for finding the distance to the stars by trigonometry, and
why the actual measurement proved so difficult." Elisabeth Lickindorf,
QUEST, Science for South Africa.

"In Proxima, Ian Glass has managed well to convey the excitement of a
scientific discovery in progress. This book is a valuable contribution to
our understanding of a small but significant aspect of the history of
astronomy and of our attempts to establish our place in the Universe. It now
forms part of the record of the history of science in South Africa." M.
Soltynski, MNASSA.

"Proxima: The Nearest Star ... is a charming little book and seems
designed primarily for the interested layman, yet it will appeal equally to
astronomers with a passion for the history of our discipline. I thoroughly
recommend it as a valued and eminently affordable addition to your
bookshelf." Wayne Orchiston, Journal of Astronomical History and
Heritage.